The Stinger missiles really made things hellish for the Soviets. No longer could they just fly their helicopter gunships with impunity. The fact that the Soviets also decided to stay and try to rule also messed things up. It is easier to win a war and go home than it is to fight one and stay. Far easier.
True, but the intent of the soviets was to add another country to their orbit. Would the regime they set up have lasted if they declared victory and left?